It’s like trying to compress a file that was generated by a random device —
Gretta: You can’t losslessly compress a truly random file.
I don’t think this is strictly true. You can’t a priori build a compression scheme that will work for an arbitrary random file (No Free Lunch Theorem). But you can ex post identify the particular patterns in a particular random file, and pick a compression scheme that picks up on those patterns. You probably end up with a pretty ugly scheme that doesn’t generalize, and so is unsatisfactory in some aesthetic sense. Especially if you’re going for lossless compression, since there’s probably a ton of noise that’s just very hard to compress in an elegant / generalizable way.
I guess the problem with allowing ex post scheme choices is you can get extreme overfitting—e.g. the compressor is just a full representation of the pattern. Useless! But if you allow lossiness, you can probably get something that looks okay on some “elegance” prior—e.g. try a Fourier series, a Taylor series, and N other “natural” decompositions and pick the one that requires the fewest bits to store.
Analogously: you need a bunch of goofy epicycles to explain canon HP magic, and especially if you’re really trying hard for an exact match maybe you just end up with something extremely unaesthetic (a ton of random bits at the end that you just have to fiat in). In the degenerate case you get Eliezer’s “we’re selecting a universe that matches what JKR wrote.” But if you’re willing to take some predictive loss in return for a simple/elegant model, you can get something like “nested nerfing” that’s decent at prediction and at elegance.
In context, I guess your claim is: “if the ‘compressor’ is post-hoc trying a bunch of algorithms and picking the best one, the full complexity of that process should count against the compressor.” Totally agree with that as far as epistemology is concerned!
But I don’t think the epistemological point carries over to the realm of rational-fic.
In part that’s because I think of JKR-magic as in fact having a bunch of structure that makes it much easier to explain than it would be to explain a truly randomly-generated set of spells and effects (e.g. the pseudo-Latin stuff; the fact that wands are typically used). So I expect an retrofitted explanation wouldn’t be crazy tortured (wouldn’t require having a compression process that tests a ridiculous number N of patterns, or incorporates a ridiculous amount of fiat random bits).
In part I’m just making a tedious “nerds have different aesthetic intuitions about stuff” point, where I think a reasonably simple well-retrofitted explanation is aesthetically very cool even if it’s clearly not the actual thing used to generate the system (and maybe required a bunch of search to find).